1. Field of the Invention
The present invention relates to an overload preventing system for a kneading machine which is for pulverizing and kneading rubber, plastics, etc.
2. Description of the Prior Art
In a kneading machine for pulverizing and kneading rubber, plastics, etc. (hereinafter referred to as "material to be kneaded, an overload is exerted on kneader blades and also on a drive means for the kneader blades in a pulverizing step of a pulverizing and kneading process for the material to be kneaded. To prevent such overload there has been proposed an overload preventing system for a kneading machine in which a load imposed on the above drive means is detected, and in response to the detected load a compressed fluid present at an upper side, or a lower side, in the interior of a lift cylinder for a weight which closes an upper opening of a case, is discharged into the atmosphere. As an example, this conventional overload preventing system will be described below with reference to FIG. 3.
In FIG. 3, a kneading machine to be prevented from overload by the overload preventing system shown therein is provided with a cylinder 51 which has at respective upper and lower positions an upper port 52 and a lower port 53 for inflow and outflow of compressed air. A piston 54 is slidably fitted inside the cylinder 51. To the underside of the piston 54 is fixed a piston rod 55 projecting downwards from a lower central part of the cylinder 51, and to the lower end of the piston rod 55 is fixed a weight 56 which is moved up and down by the piston 54. Under the cylinder 51 is disposed a frame 57 having an upper surface fixed to the lower surface of the cylinder 51 with the weight 56 being slidably fitted inside the frame 57. The frame 57 is provided exteriorly thereof with an inlet opening 58 for introducing a material to be kneaded into a case 59 which is fixed to the lower surface of the frame. An upper opening 60 of the case 59 is closed and opened by the weight 56 as the weight moves up and down. Within the case 59 are disposed kneader blades 61 of the kneading machine to pulverize and knead the material fed into the case, the kneader blades 61 being driven by an electric motor 62. For the kneading machine of the above construction, an upper pipe 64 is disposed, extending from a feed port 63 which is for the feed of compressed air up to the upper upper port 52. In the pipe 64 there is mounted an upper solenoid valve 65 which functions to respectively establish communication and cut off communication of the pipe 64 with respect to the with the upper port 52 and also functions to open the cylinder 51 side of the upper pipe 64 to the atmosphere. Also mounted in the upper pipe 64 is a pressure regulating valve 70 in a position closer to the feed port 63 with respect to the solenoid valve 65, the pressure regulating valve 70 functioning to regulate to a certain level the pressure of compressed air fed from the feed port 63 to the cylinder 51. Further, a lower pipe 66 is disposed, extending from the feed port 63 to the lower port 53, and in the lower pipe 66 there is mounted a lower solenoid valve 69 which functions to respectively establish communication and cut off communication of the feed port 63 with the lower port 53 and also functions to open the cylinder 51 side of the lower pipe 66 to the atmosphere. A meter relay 72 detects an electric current fed to the motor 62 when the motor drives the kneader blades 61 and operates the upper solenoid valve 65 reversibly in accordance with the detected current value.
In the above construction, with the weight 56 occupying an upper position in the frame 57, when a material to be kneaded is fed into the case 59 from the inlet opening 58, the upper solenoid valve 65 is operated to establish communication of the feed port 63 with the upper port 52. Then, compressed air the pressure of which has been adjusted to a predetermined certain level by the pressure regulating valve 70 is fed from the feed port 63 to the upper portion in the interior of the cylinder 51, so that the weight 56 is brought down to close the upper opening of the case 59. This closing force must be strong for the following reason. If the weight 56 should be pushed up easily by the force of a large lump of the material to be kneaded which is pulverized and moved by the kneader blades 61, the internal volume of the case 59 increases and hence the degree of freedom of the material to be kneaded increases, resulting in that the pulverizing and kneading efficiency for the material is deteriorated.
Pulverization of the material fed into the case is started by the kneader blades 61 which are driven by the electric motor 62. When the kneader blades 61 bite in a large lump of the material to be kneaded, an overload is exerted on both the kneader blades 61 and the motor 62. If no countermeasure is taken in this state, the kneader blades 61 and the motor 62 may be damaged. To prevent such overload, the electric current flowing through the motor 62 is detected by the meter relay 72, and when the detected current value exceeds the rated value of the motor 62, the exciting current for the upper solenoid valve 65 is cut off by the meter relay 72. As a result, the communication of the feed port 63 to the upper port 52 is cut off and the upper port 52 is opened to the atmosphere, whereby the compressed air present in the upper portion of the interior of the cylinder 51 is discharged into the atmosphere. Consequently, the closing force for the upper opening of the case 59 now corresponds to only the weight of the weight 56, so that a large lump of the material to be kneaded is pushed up with the operation of the kneader blades 61 and the internal volume of the case 59 increases, thus facilitating the movement of the material being kneaded, and hence the load imposed on the motor 62 is reduced. When the electric current flowing through the motor 62 has become smaller than the rated current value of the motor, the interception of the exciting current for the upper solenoid valve 65 is discontinued to reestablish communication of the upper port 52 with the feed port 63, whereby the passage leading to the atmosphere is cut off. Consequently, compressed air flows into the upper portion of the interior of the cylinder 51, so that the weight 56 strongly closes the upper opening 60 of the case 59. Thus, the reduction and reestablishment of the closing force of the weight 56 for the upper opening 60 of the case are repeated successively, whereby a large lump of the material to be kneaded is pulverized. Then, the operation shifts to the kneading step. The material to be kneaded in the kneading step is already of fine particles, so the kneader blades 61 and the motor 62 will no longer undergo an overload. When the kneading of the material to be kneaded is over, the exciting current for the upper solenoid valve 65 is cut off by a known means, whereby the compressed air present in the upper portion of the interior of the cylinder 51 is discharged into the atmosphere. At the same time, the lower solenoid valve 69 is energized by a known means to establish communication of the port 53 with the feed port 63 and cut off the passage leading to the atmosphere. Consequently, compressed air flows into the lower portion of the interior of the cylinder 51 from the feed port 63, thus causing the weight 56 to move upward. Now the pulverizing and kneading process for the material to be kneaded is over.
The kneading machine equipped with the overload preventing system according to the prior art described above in connection with FIG. 3 is useful in its own way and so is in wide use in the processing industry for rubber, plastics, etc. However, the following problems are still involved therein.
According to the above conventional overload preventing system, the depressing force for the weight to close the upper opening of the case is reduced by discharging into the atmosphere the compressed air of a volume corresponding to substantially the entire volume of the upper portion of the cylinder interior. Since this operation is repeated in the material pulverizing step, the amount of compressed air consumed is very large, thus resulting in a considerable energy loss.
Further, after the reduction of the closing force of the weight for the upper opening of the case, the compressed air is again fed to the upper portion of the interior of the cylinder through the upper solenoid valve, so it takes time for the closing force of the weight to reach a required closing force for not causing deterioration of the pulverizing efficiency. This time corresponds to a so-called time lag, which is large in the conventional overload preventing system described in connection with FIG. 3. Consequently, the time period in which a sufficient pulverizing force is not exerted on the material to be kneaded becomes long, thus leading to deterioration of the pulverizing efficiency.